Should you collimate an SCT with a star diagonal installed or in a "straight-through" configuration with an eyepiece inserted directly into the visual back? There is no doubt a diagonal can affect collimation if its mirror or prism is misaligned. If the diagonal is rotated to a viewing position different from the one it was in when col-limation was done, any alignment error may make cause collimation to be "off" at the new position. Worse, if the diagonal is removed to take pictures through the telescope in straight through fashion, the SCT may then be way out of adjustment.
Collimate with a diagonal or not, then? First, determine whether the diagonal has problems. If its prism's or mirror's alignment is not right, the image of a star will move a considerable distance in the field of an eyepiece when the diagonal is rotated to new observing positions. One possible solution is to purchase a high-quality star diagonal. The stock units that come with new telescopes are often of low quality. Even high quality units will show some image movement, however, because of a particular telescope's particular mechanical alignment characteristics rather than problems with the diagonal. Luckily, most diagonal alignment problems are too small to have much effect on collimation. Thus, it's best to collimate with the diagonal if observing will be done with the diagonal. If the scope is mostly used for imaging in straight-through fashion, collimate that way.
Another frequent question novices ask is, "How often should I collimate?" The answer is, "As often as necessary." Check collimation on a regular basis. A quick glance at a slightly out of focus star will instantly show whether the CAT needs attention or not. An SCT that doesn't go on many road trips may go months or even years before anything more than minor touchups is needed. If collimation is needed before every observing run, something is wrong. Usually the secondary is loose because collimation screws were loosened to adjust collimation before their opposite numbers were snug. Will a brand-new SCT need adjustment? Probably. Even if it was precisely collimated at the factory, a long trip and rough handling likely ensures it needs adjustment.
MCT Collimation A Rumak -style Maksutov Cassegrain with a "separate" secondary mirror holder is collimated just like an SCT. Adjust the secondary mirror mount's three screws while observing the diffraction rings of a star. In a Gregorystyle MCT , like a Meade ETX, the secondary is a silvered spot on the inside of the corrector plate, and there's no obvious way to adjust anything. Normally, collimation isn't required for these telescopes. A few Gregory Maks—including new and expensive ones—do develop collimation problems. In most cases, a new or old Gregory that's out of collimation should go back to the dealer. Although these scopes can be adjusted, they often have to be partially disassembled to do so. Properly collimating a Gregory MCT is not easy and frankly calls for an optical bench and plenty of know-how.
You probably shouldn't even dream of hacking into a Questar or other expensive MCT, but let's say you find a dirt-cheap ETX or Synta/Orion MCT on a swap table at a local star party. The price is right, but the collimation is off. You might consider a little tweaking in that case. The ETX, the Syntas, and other Gregorys are collimated by adjusting the primary mirror. The Syntas are comparatively easy to collimate, since their primary mirror adjustment screws (Allen screws) are exposed on the back of the rear cell. The secret to success is working very slowly and very carefully and being obsessive about re-entering the target star between tweaks.
The ETX and quite a few other Gregory MCTs have their mirror adjustment screws hidden under the rear cell cover and eyepiece holder and are much harder to work with. Removing the cover, which often requires removing focuser knobs and other hardware first, will expose three pairs of push-pull bolts on the primary mirror cell. One bolt of each pair is a locking screw and the other is the adjustment screw. A little careful experimentation may be required to decide which is which. When that is sorted out, loosen the three lock screws slightly and proceed to collimate the scope on a target star's donut and diffraction rings.
Unfortunately, that's easier said than done. The rear cell cover and eyepiece holder will have to be replaced after each collimation tweak so an ocular can be inserted to check the star. When the star is a perfect bulls-eye, the locking screws should be tightened down sequentially and evenly, and the scope given a final check on the star. Likely, locking down the mirror has thrown collimation slightly off, and the cover will have to be removed and the lock screw tensions fiddled with until collimation is "in" again Think long and hard before attempting to collimate any Gregory Maksutov. I can almost guarantee an unpleasant and unsuccessful experience. Even if it's "just" a 90mm ETX, send it back to the factory for collimation.
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